5-Lipoxygenase is the first dedicated enzyme in the pathway leading to the biosynthesis of leukotrienes (Samuelsson, B., Science, 120:568 (1983); Hammarstrom, S., Annual Review of Biochemistry, 52:355 (1983)). This important enzyme has a rather restricted distribution, being found predominantly in leukocytes and mast cells of most mammals. Normally 5-lipoxygenase is present in the cell in an inactive form; however, when leukocytes respond to external stimuli, intracellular 5-lipoxygenase can be rapidly activated. This enzyme catalyzes the addition of molecular oxygen to fatty acids with cis,cis-1,4-pentadiene structures, converting them to 1-hydroperoxy-trans,cis-2,4-pentadienes. Arachidonic acid, the 5-lipoxygenase substrate which leads to leukotriene products, is found in very low concentrations in mammalian cells and must first be hydrolyzed from membrane phospholipids through the actions of phospholipases in response to extracellular stimuli. The initial product of 5-lipoxygenase action on arachidonate is 5-HPETE which can be reduced to 5-HETE or converted to leukotriene A4 (LTA4). This reactive leukotriene intermediate is enzymatically hydrated to LTB4 or conjugated to the tripeptide glutathione to produce LTC4. LTA4 can also be hydrolyzed nonenzymatically to form two isomers of LTB4. Successive proteolytic cleavage steps convert LTC4 to LTD4 and LTE4.
Other products resulting from further oxygenation steps have also been described (Serhan, C. N., Hamberg, M., and Samuelsson, B., Proceedings of the National Academy of Sciences, USA, 81:5335 (1985); Hansson, G., Lindgren, J. A., Dahlen, S. E., Hedqvist, P., and Samuelsson, B. FEBS Letters, 130: 107 (1984)).
Products of the 5-lipoxygenase cascade are extremely potent substances which produce a wide variety of biological effects, often in the nanomolar to picomolar concentration range. (Sirois, P., Advances in Lipid Research, R. Paoletti, D. Kritchevesky, editors, Academic Press, 21: 79 (1985).
The remarkable potencies and diversity of actions of products of the 5-lipoxygenase pathway have led to the suggestion that they play important roles in a variety of diseases. Alterations in leukotriene metabolism have been demonstrated in a number of disease states including asthma, allergic rhinitis, rheumatoid arthritis and gout, psoriasis, adult respiratory distress syndrome, inflammatory bowel (Crohn's) disease, endotoxin shock, and ischemia-induced myocardial injury.